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Physics 4 – April 12, 2019 Do Now – Hand in the PhET acivity, Check out these links =PLX2gX-ftPVXVfoaIeiZcVZcHyeSpdkHKo Another great review guy for B.2 Thermo here s?view=50&sort=dd&shelf_id=4 Here’s his clearing house for all of his physics videos. It looks great.
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Objectives/Agenda/Assignment
Do Now – Complete and hand in the PhET activity Objective: 12.1 Matter and Energy Assignment: Make a Study plan for all 8 chapters and B.1, B.2. Watch 2 summary videos about duality Agenda: Wave/particle duality Momentum of light Matter wavelengths Electron diffraction
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Photoelectric Effect Experimental result:
The intensity of the light source had no effect on the kinetic energy of the photoelectrons. The intensity of the light source does affect the current. Kinetic energy of the photoelectrons depends on the frequency of light used. Below a certain frequency, the photoelectric effect does not occur at all. There is no significant delay (less than 10-9 s) between the light source activation and the emission of the first photoelectrons.
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Photoelectric effect hf = + Ek or E = hf –
The energy of the photon of light must be greater than or equal to the work function, , of the metal. Any excess energy becomes the kinetic energy of the electron that is ejected from the metal. E is measured with the stopping voltage for a given metal and frequency. Ek is then eV, the charge on an e times the stopping voltage, gives J, then convert to eV (Hint: Vs in Volts and E in eV will be equal but opposite values. )
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How the quantity is perceived
Wave particle duality How the quantity is perceived Particles Waves Type of quantity Matter Atoms Stoichiometry Lots of experiments!!! Light Photoelectric effect – Light on metal ejects electrons depending on frequency, not intensity (also momentum) Electromagnetic Radiation Ray diagrams, Reflection, refraction, mirrors, lenses Double Slit Expt – Proves waves
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Momentum of light Even though photons have no mass, they nevertheless have momentum. Some observable evidence of this is the momentum that is delivered to a space sail from “solar wind”. Solar wind is light. But that light can collide with a sail and deliver some momentum, even though light is massless. For a massless particle, this is E = pc. p = E/c = hf/c (c=f so 1/ =f/c) p=h/ The momentum of light depends on the wavelength of light.
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How the quantity is perceived
Wave particle duality How the quantity is perceived Particles Waves Type of quantity Matter Atoms Stoichiometry Lots of experiments!!! ????? Light Photoelectric effect – Light on metal ejects electrons depending on frequency, not intensity (also momentum) Electromagnetic Radiation Ray diagrams, Reflection, refraction, mirrors, lenses Double Slit Expt – Proves waves
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Electron beam and double slits
Gc Dr. Quantum – Double Slit Experiment Hard to visualize. This cartoon does a great job of running the virtual experiments and explaining the results.
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How the quantity is perceived
Wave particle duality How the quantity is perceived Particles Waves Type of quantity Matter Atoms Stoichiometry Lots of experiments!!! Double slit experiment works for beams of electrons!!! Light Photoelectric effect – Light on metal ejects electrons depending on frequency, not intensity (also momentum) Electromagnetic Radiation Ray diagrams, Reflection, refraction, mirrors, lenses Double Slit Expt – Proves waves
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Matter waves DeBroglie proposed that particles have a wave nature and a corresponding wavelength given by 𝝀= 𝒉 𝒑 = 𝒉 𝒎𝒗 Echoes the momentum of light: p=h/ So why don’t we observe matter waves?? What is the wavelength associated with a proton moving at 7.3 x 106 m/s, near the speed of light? What is the wavelength for a 50 kg person moving at 25 m/s?
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Uncertainty principle
With Newtonian mechanics, if we know the initial conditions of a physical system, we can calculate the conditions of the system at some later time. Recall all of the kinematics, dynamics and energy calculations we have done to do this. We cannot do the same with small particles, because of their wave duality. We can’t exactly know the position and momentum of a particle. There will always be a finite uncertainty, a theoretical limit to how good your measurements can be: 𝚫𝒙𝚫𝒑≥𝒉/𝟒𝝅 Also 𝚫𝑬𝚫𝒕≥𝒉/𝟒𝝅 visualizing a “wave particle” and how the uncertainty principle is an inherent feature. (1st few min)
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How the quantity is perceived
Wave particle duality Contains links to high quality summary videos below How the quantity is perceived Particles Waves Type of quantity Matter Atoms Stoichiometry Lots of experiments!!! Double Slit Expt with electrons Wavelength of matter QM Wavefunctions – orbitals Uncertainty principle Light Photoelectric effect – Light on metal ejects electrons depending on frequency, not intensity (also momentum) Electromagnetic Radiation Ray diagrams, Reflection, refraction, mirrors, lenses Double Slit Expt – Proves waves
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Exit slip and homework Exit Slip – none
What’s due? (homework for a homework check next class) Develop Study Plan for IB Test May 17 and May 20 Watch the two links on the previous slide Google “Quantum Mechanics made easy parts 1 and 2” to find them What’s next? (What to read to prepare for the next class) Read 7.3
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